Naturally occurring proteins are widely used for research and clinical purposes. While such proteins may be obtained from their natural source, recombinant techniques can permit the production of these proteins from non-natural sources. For example, fermentation of microorganisms constructed via recombinant technology, such as transformed bacteria, produce large quantities of human interferon at a substantially lower cost than is possible utilizing natural sources. Such recombinant DNA techniques have also been utilized to produce other important proteins, such as insulin and tissue plasminogen activator.
Bacteria altered by recombinant techniques, however, also produce contaminants and structural isoforms of the protein intended to be produced. These contaminants and isoforms include oligomeric proteins and reduced protein isoforms (see U.S. Pat. No. 4,765,903 to D'Andrea et al.), cell debris and viruses (see U.S. Pat. No. 4,732,683 to Georgiadis et al.) and pyruvate-linked isoforms (see Rose et al., J. Biol. Chem. 287:19101 (1992); Prome et al, J. Biol. Chem. 266:13050 (1991); Stevens et al, J. Biol. Chem. 252:2998 (1977); and Shapiro et al, J. Biol. Chem. 255:3120 (1980)). It is desirable to remove these contaminants during purification of the protein.
Clearly, these protein isoforms reduce the purity of the desired protein and the processes for removal of the isoforms reduce the overall yield. If, however, the protein isoforms can be converted to the desired protein, their removal is unnecessary and the overall protein yields would be significantly increased. What is needed is a way to identify undesired protein isoforms and convert them to the desired protein. The present invention addresses such needs.